Heat-sensitive recording materials, which are well known, employ a color forming reaction between a colorless or lightly-colored leuco dye and an organic or inorganic developer by which both these color developing materials are brought into contact with heat to produce a recorded image.
Such heat-sensitive recording materials are relatively inexpensive, and the recording devices are compact and easily maintained. For these reasons, they are used as recording materials in a broad range of fields such as facsimile systems, computers, and the like.
One of the most important qualities in such heat-sensitive recording materials is water resistance, which prevents a heat-sensitive recording layer from coming off when water comes in contact, and inhibits discoloration of recorded portions by the adhesion of water.
Further, it is demanded that heat-sensitive recording materials should not be occurred with sticking, i.e. have good anti-sticking properties, when used for printing with a printer.
Furthermore, recent expansion of the usage of such recording materials for POS labels, merchandise distribution labels and the like has led to demands that recording materials should have good printability, e.g. ink density (ink receptivity), ink density homogeneity (uniformity of the print), etc.
A known technique for imparting water resistance to heat-sensitive recording layers uses as binders a carboxy group-containing aqueous polymer and an oxazoline group-containing aqueous polymer (see patent document 1). Also known for use as a binder for a heat-sensitive recording layer is a certain amount of a water-dispersible polymeric substance having a softening point of from 150 to 260° C. obtained by polymerizing mainly at least one hydrophobic vinyl monomer having a glass transition temperature of 55° C. or higher in the presence of a polymer latex having a glass transition temperature of 50° C. or lower (see patent document 2). The use of a water-soluble modified polyvinyl acetal resin having specific hydrophilic groups is also known (see patent document 3). It is further known that a polymer emulsion obtained under specific conditions by emulsion polymerizing a monomer containing (meth)acrylonitrile, (meth)acrylic ester and ethylene-unsaturated carboxylic acid can be used as binders (see patent document 4). It is known that 70% or more of the binder used in a heat-sensitive recording layer may be an acrylic ester copolymer (see patent document 5). It is also known that a heat-sensitive recording layer may contain a copolymer of a vinyl monomer having a heterocyclic group with a basic nitrogen atom in its ring with an ethylene-vinyl monomer whose α,β double bonds have been saturated (see patent document 6). It is known that a heat-sensitive recording layer may contain an acrylic emulsion and colloidal silica, and further contains an inorganic pigment of a specific particle diameter (patent document 7).
However, these techniques did not always render sufficient water resistance or printability.
For the purpose of improving water resistance, the use of at least one crosslinking agent, such as diols and aldehydes is known (see patent document 8).
However, when a crosslinking agent is used, there are problems in that crosslinking requires a long time, the productivity of the heat-sensitive recording materials is reduced, and/or the like.
To eliminate such problems caused by crosslinking agents and enhance water resistance concurrently, the use of specific copolymer resin emulsions and polyolefin copolymer resin emulsions for a protective layer is known (see patent documents 9 and 10). However, further improvement in properties such as heat-sensitivity, etc., is still demanded.
To enhance water resistance and anti-sticking properties, it is known that a heat-sensitive recording layer may contain emulsion-dispersed stearamide and a complex between colloidal silica and an acrylic polymer or styrene-acrylic polymer (see patent document 11). It is known that a heat-sensitive recording layer may contain as binders casein and a hydrophobic acrylic resin having an average molecular weight of 5 million or greater (see patent document 12). To impart outstanding water resistance and printability, the use as binders of a silicone-modified polyvinyl alcohol and an aqueous graft copolymer of silicone and a polymer containing ethylenically-unsaturated carboxylic acid is known (see patent document 13).
While the water resistance, anti-sticking properties and printability attained by these techniques are satisfactory, further improvement is still desirable.
[Patent document 1] Unexamined Japanese Patent Publication No. 1994-155916
[Patent document 2] Unexamined Japanese Patent Publication No. 1994-206376
[Patent document 3] Unexamined Japanese Patent Publication No. 1994-344668
[Patent document 4] Unexamined Japanese Patent Publication No. 1996-337057
[Patent document 5] Unexamined Japanese Patent Publication No. 2001-277719
[Patent document 6] Unexamined Japanese Patent Publication No. 2003-94806
[Patent document 7] Unexamined Japanese Patent Publication No. 2004-25775
[Patent document 8] Unexamined Japanese Patent Publication No. 2002-29155
[Patent document 9] Unexamined Japanese Patent Publication No. 2004-74531
[Patent document 10] International Patent Publication No. 2004/016440
[Patent document 11] Unexamined Japanese Patent Publication No. 1997-207435
[Patent document 12] Unexamined Japanese Patent Publication No. 1998-272839
[Patent document 13] Unexamined Japanese Patent Publication No. 1999-227336